1. Field of the Invention
The invention relates to a self-centering brake assembly incorporating a brake drum and an automatic adjusting mechanism which maintains a centered relation of the brake shoes in reference to the brake drum.
2. Description of the Prior Art
The present invention utilizes a torque spider, i.e. two torque plates which are in flat face-to-face engagement at their inner brake mounting portion and are axially spaced apart at their outer peripheral portions. A primary brake shoe and a secondary brake shoe are connected together at one pair of adjacent ends through an automatically adjustable strut and at the opposite pair of adjacent ends, the brake shoes are selectively spreadable into engagement with the brake drum by means of a wheel cylinder. The actuated ends of the shoes are also the anchored ends of the shoes, with the anchoring occurring on selected abutments of the two torque plates. In one direction of drum rotation the two brake shoes anchor jointly on one of the anchoring abutments, and when the brake drum reverses rotation, the two brake shoes, acting as a shoe ring, anchor jointly on the other spreadable end of the shoe ring. The adjacent spreadable ends of the shoes each carry an anchor pin, and the anchor pins are received in complementary anchoring recesses of the two torque plates. The expandable strut which connects the articulated ends of the shoes includes a serrated adjuster wheel, an adjuster lever, an adjuster lever return spring and an overtravel spring. The automatic adjuster functions during reverse braking by rotating a serrated turnable wheel to lengthen the strut and thus compensate for overtravel of the secondary brake shoe.
The problem to be overcome by the present invention is that the force matrix developed by the adjuster lever return spring and the overtravel spring which acts on the adjuster lever tends to cause an unbalance in the shoe ring when the brakes are unactuated or at rest that biases the shoes against the drum and mispositions the shoe ring from concentricity with the drum.
The problem addressed by the present invention is that the force system acting upon the shoes during brake adjustment must function to adjust the brake shoes, but once the actuating force developed upon the shoes, as well as the anchoring forces, are relieved, the spring forces which are component parts of the automatic adjuster must bias the shoe ring into a retracted position concentric with the drum. The described operation will result only when there is a balance of forces wherein moments taken about the anchor pins are self canceling to equal approximately zero. The strategy of the present invention is to balance the moments about each of the anchor pins so that the sum of these moments equal approximately zero because of the arrangement of the force system, consisting of the adjuster lever return spring and wire form link. It has been found by accurately analyzing and then disposing the components of the adjuster, i.e. the lever, the adjuster lever return spring, wire form link, and overtravel spring, it is possible to develop an accurate location for these components with the result that the sum of the moments about each of the anchor pins is substantially zero.
A distinct advantage of the present invention is that the application adjuster of the present invention has a reduction in the number of component parts from five or six, which is typical of that of the prior art, to a total number of four making it easier to assemble and to follow the contour of the brake drum. Because the brake follows the contour of the brake drum, it permits larger hub areas for front brake drum applications and conversely enables smaller duo-servo brakes on the rear of the vehicle. It should be understood that while the present invention finds its maximum utility with a due-servo double anchor torque spider brake, it also finds application in a duo-servo single anchor design with the backing plate being suitably modified for such installation.